1. Field of the Invention
The invention relates to a improved process for removing post-RIE polymer rails formed on a Al/Cu metal line of semiconductor structures or microelectronic devices using a chemical mixture that contains an etching gas and an acid neutralizing gas. More specifically, the improvement process comprises removing the sidewall polymers left behind after the metal (Al/Cu) RIE process, using a mixture of an etching agent and an acid neutralizing reactant introduced either in a gaseous or plasma form, wherein the etching agent is a fluorine based species such as HF, and the acid neutralizing reactant is a chemical such as NH3.
2. Description of the Prior Art
In ULSI (ultra large scale integration) devices, the electrical connection between metal layers is achieved by sub micron via holes. These via holes are formed by cutting through interlayer dielectric (ILD) using anisotropic reactive ion etching (RIE) processes, usually with CF4 or CHF3 and with inert gas mixtures. The etch residue is non-corrosive and is easily removed.
During the RIE process of a composite structure having silicon oxide as the interlayer dielectric, a barrier layer, a metal stack layer, and a photo-resist, after exposure and development of the photo-resist, insoluble polymers that are mostly inorganic in nature form as sidewalls and some of the chemical constituents are Al,Si,Ti,O, C and Cl. This residue is relatively corrosive and difficult to remove.
Currently, post metal RIE cleaning is done using a chromic-phosphoric acid tank bath or by other solvent based chemistry. Further, there is a certain time window of approximately 4 hours between completion of the RIE process and the wet cleaning step to avoid corrosion.
Some drawbacks of the currently existing wet chemical cleaning methods are as follows:                1) An acid based chemistry (without any HF in it) is not efficient in removing polymer rails that have high silicon content in them (as found in metal pads located in the support region); and        2) A solvent based chemistry generally takes a longer process time (approximately 10 minutes as compared to 2 to 4 minutes for a typical acid based clean) and typically is encumbered as a result of costs and environmental disposal requirements.        
An apparatus for and method of accurately using a gaseous mixture of HF and NH3 to etch precise amounts of silicon dioxide is disclosed in U.S. Pat. No. 5,282,925. This method comprises: admitting a reactant containing gas into a vacuum chamber onto a substrate having a material to be etched to form a film of the reactant on the surface of substrate material; controlling the composition and residence time of the film on the surface of the material to be etched in an amount accurate of material on the substrate; and removing any unwanted reactant and reaction product from the chamber or surface of the substrate. The patent does not teach reaction with a complex mixture of silicon dioxide, aluminum dioxide, and carbon, or successful diffusion of ammonia and HF through the reaction products of the complex mixture.
U.S. Pat. No. 5,129,958 discloses a method for cleaning a CVD deposition chamber in a semiconductor wafer processing apparatus comprising, contacting fluorine residues remaining therein from a plasma fluorine cleaning step with one or more reducing gases.
Pages 235-242 of the Electrochemical Society Proceedings Volume 95-20 by Tong et al., discloses removal of polymeric/silicate residues and reduction of contact resistance for inter-metal via holes by vapor phase HF cleaning. The vapor HF treatment reduces the metal-to-via contact resistance, due to the presence of AlF3, formed by the reaction between the vapor HF and the Al/Al2 O3 at the via base. CF4 or CHF3 based RIE chemistry on an oxide field produces a polymer of low aluminum content without chlorides. Such a polymer is easily removed and is less corrosive than the one formed after aluminum RIE.
A process for removal of residues remaining after etching a polysilicon layer in formation of an integrated circuit structure is disclosed in U.S. Pat. No. 05,296,093, wherein, in lieu of removing polymerized silicon/oxide-containing residues by dipping previously RIE etched structure into a HF bath, the RIE etched structure is treated with an aqueous ammonium-containing base/peroxide solution after completion of the RIE etch to remove such residues.
There is a need in the art of cleaning RIE etched materials to lessen the time window of about 4 hours between the completion of the RIE process and the wet clean step.
Putting it another way, there is a need to avoid the time coupling between RIE and wet clean and to avoid the use of a wet chemical clean in the art of reactive ion etching (RIE) processes which form metal lines by cutting through metal stack by virtue of anisotropic reactive ion etching (RIE) processes.